Recently, a new Multiple-Input-Multiple-Output (MIMO) system called the Full-dimensional MIMO (FD-MIMO) has shown great potential to improve the spectral efficiency of the current Long Term Evolution (LTE) networks. FD-MIMO applies a large number of active antenna elements arranged in a two-dimensional lattice at the Base-Station (BS). As such the BS array is capable of beamforming in azimuth and elevation providing sufficient degrees of freedom to support Multi-User MIMO (MU-MIMO). The operation frequency of base stations depends on spectrum availability, service provider and duplex scheme used. For example, LTE TDD bands #41 (2.496-2.69 GHz) and #42 (3.4-3.6 GHz) and FDD bands #7 (2.5-2.57 GHz UL and 2.62-2.69 GHz DL) and #22 (3.41-3.5 GHz UL and 3.51-3.6 GHz DL) provide suitable spectrum for FD-MIMO. At these frequency bands, wavelengths are in the range of 8-12 cm and therefore antenna systems are relatively bulky, considering that FD-MIMO systems could be comprised of hundreds of active antennas. As such a high degree of integration is necessary to maintain an overall small form factor, low cost, light weight, and avoid unnecessary power loss. This means that the several boards that make up the active antenna system such as the antenna board, the antenna feed board, the transceiver board, and baseband board need to be integrated in one compact unit.
Typically, integrating a transceiver and baseband boards requires multilayer PCB technology and extremely efficient system architecture. However, antenna and antenna feed board integration is not straightforward because it typically results in loss of bandwidth and efficiency.